Intersex pride flag, created by Organisation Intersex International Australia in 2013.

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Intersex refers to any biological ambiguity of an animal or human's sex from "chromosomes, gonads, sex hormones, or genitals". Previously, intersex was synonymous with hermaphrodites; however, this has fallen out of use because it does not correctly describe the the condition.[1] (It wouldn't be surprising if it reaches "homosexual" status in terms of being offensive in the future")

Intersex can be distinguished from transsexualism, in that intersex individuals have natural physical sex ambiguity; whereas any ambiguity in a transsexual's body is artifical. Like transsexual individuals, intersex individuals sometimes come to feel they were assigned the wrong gender at birth, and seek to change that assignment; this is generally distinguished from transsexualism, though it tends to be used to support the supposition that each person has an intrinsic gender identity that cannot be wrongfully imposed.

Using Anne Fausto-Sterling's definition, 1.7% of all human births are intersex.[2]

Non-intersex humans are called perisex, or dyadic, depending on who you're talking too. (Intersex people generally prefer you use perisex though).

Right-wing, conservative and radical feminist views on various issues (e.g. gender relations, transgenderism, etc.) assume that male and female are clearly distinct. The existence of intersex people undermines that assumption of gender essentialism. There is no one single feature that makes a person female or male (despite the overly simplistic assumption of many that sex can be defined in terms of what types of chromosomes one has). In biological terms, female and male exist on a continuum, even though the vast majority of individuals exist clearly at one end or the other, and only a small minority occupy intermediate positions. The question of sex/gender is not always decidable: at the extremes of the continuum the correct answer is clear, yet there are places in the middle where the question is essentially undecidable and arbitrary.

The SRY gene, present on the Y chromosome, causes the development of testis rather than ovaries, and starts the sex differentiation process. If this gene is damaged or deleted, then a female course of development will proceed to an extent, rather than a male course. The individual will have mainly female anatomy, but will have streak gonads instead of functional ovaries or testes. The individual will not be able to produce viable sperm/eggs but with donated eggs can become pregnant.[3]

XX males

If the SRY gene somehow migrates from the Y chromosome to another chromosome, then a male course of development will ensue despite the XX karyotype.[4] Known cases are due to SRY migrating to the X chromosome. Sterile.

Complete androgen insensitivity syndrome

Normal XY karyotype, but individual has damage to genes necessary for testosterone to have its effect. As a result, the individual will undergo a mostly female development of the body. External sex organs will be female, but will have male internal sex organs (including testes instead of ovaries). High testosterone levels, above the male norm, but the body will fail to respond to testosterone in the usual male manner. Due to high testosterone levels, the body will convert some of the testosterone into estrogen, which can then cause normal female puberty, except menstruation will not occur. The typical phenotype for this syndrome is women who are extremely tall, not particularly curvy, and lacking in body hair (yes, the Western beauty ideal is very similar to the appearance of a woman who has an intersexuality syndrome).

Partial androgen insensitivity syndrome

Some deficiency in genes responsible for response to testosterone, but not total. This represents a range of conditions, at one extreme the individuals are mostly similar to normal males, but have some slight feminine characteristics and fertility issues; as the level of insensitivity increases, the result is less and less masculine, and more and more feminine. Many people with partial androgen insensitivity undergo sex reassignment therapy to make their gender more definitive.

5α-reductase deficiency

An error in the enzyme that changes testosterone to dihydrogen testosterone (DHT) results in low levels of DHT, which is a more active hormone than mere testosterone.[5]

Congenital adrenal hyperplasia(CAH)

Due to abnormal function of the adrenal glands, cortisol and aldosterone are produced insufficiently while androgens are overproduced. Classic CAH requires medical intervention as these hormone imbalances can be life-threatening for infants due to resulting electrolyte imbalances and hypertension.[6] Additionally, in females, it can cause a male external sex, or with later onset cause masculine-like puberty.

Genetic chimerism/mosaicism

In chimerism, two zygotes, one XX and one XY, fuse together before cell differentiation, embryogenesis, and implantation happen. As a result, some parts of the body have the XX genome, while other parts have the XY genome. In both cases, the phenotypical consequences (including gonadal and genital) vary widely, depending on the exact distribution of the two (or more) karyotypes throughout the body.